Since the 1990s, research on the phase contrast method using a phase difference of an X-ray beam has been conducted mainly in synchrotron radiation facilities.
Further, research on phase contrast imaging using X-ray tubes in laboratories has also been conducted, and a propagation method, the Talbot interference method, which will be described below, can be performed in principle.
In one propagation method a subject is irradiated with an X-ray beam generated by a micro-focus X-ray source, and the X-rays refracted in the test object are detected by a detector which is at a sufficient distance from the test object. With this method, an image can be acquired, which that is clearer and easier to see by enhancing the outline of a conventional absorption contrast image, but it is difficult to image soft tissue inside a test object.
Meanwhile, the Talbot interference method is a method for retrieving a phase image from an interference pattern which is expressed under certain interference conditions by using a transmission-type diffraction grating as described in U.S. Pat. No. 5,812,629.
For imaging by the Talbot interference method, an X-ray source which is spatially coherent, a phase grating for periodically modulating the phase of X-rays and a detector are, at least, required.
In order to have sufficient spatial coherence, it is necessary that λ×(R/s) satisfies the condition of being sufficiently large with respect to the pitch d of the phase grating.
Here, λ represents the wavelength of the X-rays, R represents the distance between the X-ray source and the phase grating, and s represents the size of the source. In the description, the “pitch” of the phase grating is the period at which the gratings are arranged.
This may be a distance C between the center portions between a certain grating and the grating adjacent to it, or may be a distance C′ between end surfaces of these gratings, as shown in a schematic view of the phase grating of FIG. 8.
In Talbot interference, an interference pattern reflecting the shape of the phase grating appears at a specific distance from the phase grating. This is called a “self-image”.
The position where the self-image appears is (d2/λ×n or (d2/λ)×(l/m) from the phase grating, and this position is called a Talbot position. In this case, n and m are integers.
Here, if a test object is disposed in front of the phase grating, the X-rays which are irradiated are refracted by the test object. If the self-image of the phase grating by the X-rays transmitted through the test object is detected, the phase image of the test object can be obtained.
However, in order to detect the self-image which occurs with sufficient contrast, an X-ray image detector with high spatial resolution is necessary, and therefore, imaging is performed by using an absorption grating, which is a diffraction grating made of a material absorbing X-rays and having a sufficient thickness.
That is to say, if the absorption grating is disposed at a Talbot position, which is the position where the X-rays transmitted through the phase grating form a self-image, the phase shift can be detected as deformation of moiré fringes, and therefore, if the moiré fringes are detected with an X-ray image detector, the test object can be imaged.
Incidentally, in Talbot interference, in order to satisfy the coherence condition, synchrotron radiation with high coherency, and a micro-focus X-ray tube having a source with a micro focal spot size, are used.
However, synchrotron radiation has a problem from a practical point of view. A micro-focus X-ray tube, although it can be used in a laboratory system, has a small focal spot size and, therefore, has small brilliance. Therefore, the micro-focus X-ray tube has a problem of being incapable of obtaining a sufficient brilliance depending on the purpose of imaging.
From these reasons, “Phase Retrieval and Differential Phase-Contrast Imaging with Low-Brilliance X-Ray Sources”, F. Pfeiffer et al., April 2006/Vol. 2/NATURE PHYSICS proposes an X-ray Talbot-Lau-type interferometer in which a source grating is disposed directly behind an X-ray source and Talbot interference is observed by using a normal X-ray tube.
Here, the term “source grating” means a diffraction grating having a periodical structure in one direction or two directions, and is configured by a region which transmits X-rays and a region which shields X-rays.
Further, it is necessary that the Talbot-Lau-type interferometer satisfies the following condition:g=G·l/L 
where g represents the pitch of the absorption grating for X-rays, G represents the pitch of the source grating for X-rays, l represents the distance between the phase grating for X-rays and the absorption grating for X-rays, and L represents the distance between the source grating for X-rays and the phase grating for X-rays.
According to the X-ray Talbot-Lau-type interferometer as above, Talbot interference can be observed even with use of a normal X-ray tube with low coherency.